The present invention relates generally to bulk acoustic wave resonators and filters and, more particularly, to the fabrication of resonators operated in the piston mode.
It is known that a bulk acoustic-wave (BAW) device is, in general, comprised of a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer. The fundamental resonance occurs when the wavelength of the mechanical/acoustic wave (produced by the RF signal) is about twice the thickness of the piezoelectric layer. Although the resonant frequency of a BAW device also depends on other factors, the thickness of the piezoelectric layer is the predominant factor in determining the resonant frequency. As the thickness of the piezoelectric layer is reduced, the resonance frequency is increased. BAW devices have traditionally been fabricated on sheets of quartz crystals. In general, it is difficult to achieve a device of high resonance frequency using this fabrication method. When fabricating BAW devices by depositing thin-film layers on passive substrate materials, one can extend the resonance frequency to the 0.5-10 GHz range. These types of BAW devices are commonly referred to as thin-film bulk acoustic resonators or FBARs. There are primarily two types of FBARs, namely, BAW resonators and stacked crystal filters (SCFs). An SCF usually has two or more piezoelectric layers and three or more electrodes, with some electrodes being grounded. The difference between these two types of devices lies mainly in their structures. FBARs are usually used in combination to produce passband or stopband filters. The combination of one series FBAR and one parallel FBAR makes up one section of the so-called ladder filter. The description of ladder filters can be found, for example, in Ella (U.S. Pat. No. 6,081,171). As disclosed in Ella, an FBAR-based device may have one or more protective layers commonly referred to as the passivation layers. A typical FBAR-based device is shown in FIGS. 1a and 1b. As shown in FIGS. 1a and 1b, the FBAR device comprises a substrate 110, a bottom electrode 120, a piezoelectric layer 130, and a top electrode 140. The FBAR device may additionally include a membrane layer 112 and a sacrificial layer 114, among others. The substrate can be made from silicon (Si), silicon dioxide (SiO2), Galium Arsenide (GaAs), glass or ceramic materials. The bottom electrode and top electrode can be made from gold (Au), molybdenum (Mo), tungsten (W), copper (Cu), nickel (Ni), titanium (Ti), Niobium (Nb), silver (Ag), tantalum (Ta), cobalt (Co), or aluminum (Al). The piezoelectric layer 130 can be made from zinc oxide (ZnO), zinc sulfide (ZnS), aluminum nitride (AlN), lithium tantalate (LiTaO3) or other members of the so-called lead lanthanum zirconate titanate family. The passivation layer is typically made from a dielectric material, such as SiO2, Si3N4, or polyimide, to serve as an electrical insulator and to protect the piezoelectric layer. It should be noted that the sacrificial layer 114 in a bridge-type BAW device is, in general, etched away in the final fabrication stages to create an air interface beneath the device. In a mirror-type BAW device, there is an acoustic mirror structure beneath the bottom electrode 120. The mirror structure consists of several layer pairs of high and low acoustic impedance materials, usually quarter-wave thick. The bridge-type and the mirror-type BAW devices are known in the art.
The desired electrical response in an FBAR-based device is achieved by a shear or longitudinal acoustic wave propagating in the vertical thickness through the device. Besides these wave modes, there exist other modes, known as the Lamb waves, that may deteriorate the electrical response. In quartz crystals, the strength of these spurious modes is controlled by adjusting the thickness and the width of the top electrode. In an FBAR-based device, the dimension in thickness direction is so small that it renders thickness adjustment difficult and impractical. A possible solution to the problems associated with the spurious modes is to thicken the edge of the top electrode. As disclosed in Kaitila et al. (WO 01/06647 A1, hereafter referred to as Kaitila), a frame-like structure 150 is formed on top of the top electrode 140 to thicken the edge thereof. As shown in FIGS. 1a and 1b, the frame-like structure 150 is a rectangular frame for defining a first zone and a second zone for acoustic wave excitation. The first zone is the area under the rectangular frame 150, and the second zone 148 is the area surrounded by the rectangular frame 150. With such a structure, the cut-off frequency of the piezoelectrically excited wave modes in the first zone and that of the second zone are different. When the width of the frame-like structure and the acoustic properties of the layer structure are properly arranged, the displacement relating to the strongest of the piezoelectrically excited resonance modes is substantially uniform in the second zone. Thus, the spurious resonances in the electric response of the bulk acoustic wave device are suppressed, and the FBAR is said to operate in a piston mode.
It should be noted that, as disclosed in Kaitila, the frame-like structure may be circular, square, polygonal, regular or irregular. Also, the frame-like structure can have different configurations, as shown in FIGS. 2 and 3, to achieve the piston mode. As shown in FIGS. 2 and 3, part of the piezoelectric layer 130 is covered by a passivation layer 160, and part of the passivation layer is sandwiched between the piezoelectric layer 130 and the frame-like structure 150 extended upward from the edge of the top electrode 140. In FIGS. 2 and 3, the frame-like structure 150 is basically where the top electrode 140 overlaps with the passivation layer 130. It should be noted that, FIG. 1a is a cross section view of a BAW device, as viewed in the lateral direction and the top, while FIG. 2 and FIG. 3 are cross section views of a BAW device, as viewed in the horizontal direction.
Traditionally, the frame-like structure is fabricated by forming an electrically conducting layer on top of the passivation layer and an exposed part of the piezoelectric layer, and removing part of the electrically conducting layer, as shown in FIGS. 4a-4e. In FIGS. 4a-4e, only the top few layers are shown. As shown, the device has a patterned passivation layer 160, which covers most of the piezoelectric layer 130 but leaves a section 132 of the top surface exposed. A top metal layer 128 is formed on top of the passivation layer 160 and the exposed portion 132 of the piezoelectric layer 130. The portion of the top metal layer 128 that is in direct contact to the piezoelectric layer 130 is denoted by reference numeral 148, as shown in FIG. 4b. As shown in FIG. 4c, an etching mask 200, such as a photoresist mask, is provided on top of the device. As shown in FIG. 4c, the mask 200 is skewed to the left in reference to the center portion 148. The exposed parts of the top metal layer 128 can be removed with an etching process to form an upper electrode 140, as shown in FIG. 4c. FIG. 4d shows the device after the etching mask 200 has been stripped. As shown, the upper electrode 140 has a frame-like structure similar to the structure 150 as shown in FIG. 3. However, the left section 150xe2x80x2 of the top electrode 140 is much broader than the right section 150xe2x80x3, and this is not the intended result. For example, the intended result is that the left section 150xe2x80x2 and the right section 150xe2x80x3 are substantially the same. As such, the operation of the device in piston mode may be compromised. The unintended result is due to the misalignment of the mask 200. FIG. 4c is used to illustrate the disadvantage of the traditional fabrication method, where the mask 200 is not positioned as intended. The mask 200 is intended to be symmetrically positioned with regard to the center portion 148, for example.
Even in the case when the mask 200 is correctly positioned there is a possibility that unintended results arise because of limitations in the manufacturing process. For example, the top metal 128 can be over-etched, as shown in FIG. 4f, causing a section 152xe2x80x2 to be etched away from the left section 150xe2x80x2 and a section 152xe2x80x3 to be etched away from the right section 150xe2x80x3. In this case the suppression of spurious resonances may become less effective, because the width of the frame like structure is different than planned.
Thus, it is advantageous and desirable to provide a method of producing bulk acoustic wave devices, wherein the frame-like structure on top of the top electrode for suppressing spurious resonance can be fabricated more consistently and precisely.
It is a primary object of the present invention to achieve a bulk acoustic wave device operating substantially in a piston mode, wherein a frame-like structure on the top electrode of the bulk acoustic wave device is used to reduce the spurious resonance, and wherein the width of the frame-like zone around the frame-like structure is consistent with the requirement of the piston mode. The object can be achieved by using a self-aligning fabrication process, wherein the width of the frame-like structure is substantially determined by the step of forming the frame-like structure, and not determined by an etching step thereafter. It should be noted that the frame-like zone around the frame-like structure is, in general, symmetrical, but the width of the zone may not be uniform in all cases. In some cases, the frame-like structure needs to be designed with varying width at different points of the periphery. For example, in the comer areas of a rectangular frame-like structure, it is necessary to have minor variations in the width to achieve a good piston mode. The object of the present invention is to achieve a frame-like structure substantially as designed.
Thus, according to the first aspect of the present invention, a method of producing a bulk acoustic wave device with reduced spurious resonance, wherein the device has a piezoelectric layer having a first side and an opposing second side, a first electrode layer provided on the first side of the piezoelectric layer and a second electrode layer provided on the second side of the piezoelectric layer. The method comprises the steps of:
(a) providing a frame-like structure in contact with the first electrode layer, wherein the frame-like structure has an outer boundary enclosing a first area and an inner boundary enclosing a second area;
(b) causing the frame-like structure to fuse with the contacting portion of the first electrode layer to form a fused portion, wherein the fused portion surrounds a section of the first electrode layer;
(c) providing a mask on top of the first electrode layer including the fused portion, wherein the mask has a perimeter defining a masking area larger than the second area but smaller than or equal to the first area of the frame-like structure, and the mask is positioned to cover entirely the surrounded section of the first electrode layer and at least part of the fused portion, thereby exposing remaining portion of the electrode layer outside the fused portion; and
(d) removing the remaining portion of the first electrode layer for forming a modified electrode layer.
Optionally, when the second electrode layer is the bottom electrode of the device, it is preferable to have the following steps carried out prior to providing the first electrode layer on the piezoelectric layer:
(e) providing a dielectric layer on the first side of the piezoelectric layer, wherein the frame-like structure is provided on top of the dielectric layer;
(f) providing a further mask over the frame-like structure and the dielectric layer, wherein the further mask has an inner boundary defining a clear area, which is larger than the second area but smaller than the first area of the frame-like structure, and wherein the further mask is so positioned that the inner boundary of the further mask is located between the outer boundary and the inner boundary of the frame-like structure, thereby exposing through the clear area a portion of the dielectric layer and shielding a remaining portion of the dielectric layer underlying the first mask;
(g) removing the exposed portion of the dielectric layer for exposing a portion of the piezoelectric layer defined by the inner boundary of the frame-like structure; and
(h) removing the further mask so as to allow the first electrode layer to be provided on top of the exposed portion of the piezoelectric layer and at least a portion of the dielectric layer.
Preferably, the removing of the exposed portion of the dielectric layer in step (g) is carried out by a dry etching process.
Preferably, the frame-like structure is made of nickel and the electrically conducting layer is made of aluminum to form an alloy of nickel and aluminum.
It is possible that the frame-like structure is made of silicon and the electrically conducting layer is made of titanium to form an alloy of titanium silicide.
Optionally, a hard mask is provided on the frame-like structure prior to step (f).
Preferably, the hard mask is made of aluminum nitride, and the removing of the exposed portion of the dielectric layer in step (g) is carried out by a fluorine plasma etching process.
Preferably, the dielectric layer is made of silicon dioxide (SiO2) or silicon nitride (Si3N4).
The bulk acoustic wave device can be a bulk acoustic wave resonator, a stacked crystal filter, a low frequency device, such as a single crystal resonator, or a combination thereof.
According to the second aspect of the present invention, a bulk acoustic wave device with reduced spurious resonance, wherein the device has a piezoelectric layer having a first side and an opposing second side, a first electrode layer provided on the first side and a second electrode layer provided on the second side of the piezoelectric layer. The device comprises:
a frame-like structure in contact with the first electrode layer, wherein the frame-like structure has an outer border and an inner border, and the frame-like structure is caused to fuse with the contacting portion of the first electrode layer to form a fused portion, and wherein the fused portion defines a first section of the first electrode layer within the fused portion and a second section of the first electrode layer outside the fused portion, which is removed from the piezoelectric layer.
Alternatively, the device comprises a dielectric layer provided on the piezoelectric layer and the frame-like structure provided on the dielectric layer prior to providing the first electrode layer, wherein the dielectric layer inside the inner border of the frame-like structure is removed to expose a section of the first side of the piezoelectric layer so as to allow the first electrode layer to be provided on the device in contact with the exposed section of the piezoelectric layer, the frame-like structure.
The present invention will become apparent upon reading the description taken in conjunction with FIGS. 5a-7f.